The invention relates to a vitreous material in which an especially steep refractive index gradient can be produced by means of an ion exchange process, e.g. a salt bath.
As is known, glasses of this type may be utilized for optical systems in which the refractive deflection of a light beam is not (or not exclusively) produced by any curved surfaces but instead is produced by the medium itself. The material may be utilized in the form of rods and disks, as well as in the usual lens form.
The use of ion exchange processes in order to change physical properties of glasses has been known and their theoretical prerequisites and the performance of the exchange processes have been described before, e.g. compare E. W. Marchand: Gradient Index Lenses; E. Wolf: Progress in Optics XI, North Holland 1973, pp. 306-337; W. G. French, A. D. Pearson: Amer. Ceram. Soc. Bull. 49:974-977 (1970); H. Kita, T. Uchida: Laser and Applied Ray Technology 2:39-41 (1971); DT-OS No. 24 56 894; and U.S. Pat. No. 3,873,408.
A refractive index gradient producted by an ionic exchange is influenced to a great degree by those ions involved in the interchange; relative to monovalent ions, both the atomic numbers in the Periodic System of Elements and the ion radii are of decisive influence. Accordingly, copper ions, silver ions and, to an even greater degree, thallium ions result, to give an example, in the highest refractive index gradients when used as elements or partners in the exchange process. Unfortunately, Cu.sup.+ ions nd Ag.sup.+ ions tend to cause discolorations or cloudiness in the glass while thallium salts are highly poisonous and require extensive protective measures.
It has been suggested that bivalent ions be used as elements in the exchanges. However, the diffusion constants are smaller by several orders of magnitude than those of monovalent ions, relative to the same or similar basic glasses. Hence, for suitable exchange temperatures (in the vicinity of the Tg-area of glasses) a noticible diffusion, with acceptable diffusion times (20-200 h), can exist only in the micrometer range, i.e. within a very thin surface skin.
Therefore it would be expected that, for example, bivalent ions would have no, or only a slight, effect on the production of a refractive index gradient. Surprisingly, it has now been found that certain bivalent ions have a considerable influence on the size of a refractive index gradient which has been produced by the diffusion of monovalent ions, e.g. alkali ions. The influence of these system-altering ions, which may differ in accordance with their different ion radii, is of a secondary nature since there is no, or only an inconsequential, change in their own concentration in the area of diffusion. The observed effects on the refractive index gradients produced apparently resulted from altered structural properties of the glass. While not wishing to be bound by any theory of the invention, it is presently believed that the addition of oxides of bivalent elements, in particular the alkaline earths, to silicate glasses may have unexpected effects on the diffusion properties of the alkali ions which are basically responsible for the creation of the refractive index gradient.
For the purpose of creating refractive index gradients in glasses, the preferred glasses have heretofore been the conventional aluminum silicate glasses and boron silicate glasses, which usually contain only relatively small amounts of alkaline earth oxide (usually as calcium oxide) in addition to the alkaline oxides in order to stabilize the glass. For example, definite refractive index gradients can be achieved with these glasses which contain a few percent of Li.sub.2 O, by means of a sodium salt bath and diffusion exchange. In the case of a glass rod with a diameter of 2 mm, this gradient between the rod axis and the periphery, when measured at the n.sub.d -line, generally amounts to a maximum of approximately delta n=70-80.times.10.sup.4 with a 48 hour exchange period and a bath temperature of 530.degree. C. When the exchange period is shortened to 24 hours, the delta n-value is lower. When the exchange period is extended to 192 hours, the refractive index gradient is lower, as well. The maximum gradient obtainable depends on the rod diameter. As the diameter gets smaller, the gradient which can be obtained with the same composition of the glass gets smaller. This is quite understandable, in view of the fact that the diffusion quickly penetrates to the rod axis in rods with small radii while the maximum exchange has not yet taken place on the periphery. As the radius of the rod becomes larger, higher delta n-values can be obained. However, if the radius of the glass rod becomes greater than the depth to which the exchange ions can penetrate, it is no longer possible to achieve an approximation of a preferred gradient profile, e.g., a parabola of the second order.